This invention relates to a braking pressure generator for an automotive vehicle hydraulic brake system with a booster piston to actuate a master brake cylinder. The booster piston is pressurizable by an auxiliary hydraulic pressure in a booster chamber, and a pedal actuation device cooperates with the booster piston by way of a valve device cooperates with the booster piston by way of a valve device for the control of the auxiliary pressure. A secondary cylinder chamber communicates with a working chamber of the master brake cylinder, and a hydraulically actuatable secondary cylinder piston is movable into the secondary cylinder chamber.
In a braking pressure generator disclosed in German patent application No. P 35 08 709.9 published Sept. 18, 1986 (corresponding to U.S. application Ser. No. 838,687 filed Mar. 12, 1986) a filling stage cylinder is provided for the formation of the secondary cylinder chamber. The two-stage cylinder of the filling stage cylinder is pressurizable by pressure medium displaced by the booster piston upon the booster piston plunging into an annular chamber arranged between the master cylinder and the brake booster. In this arrangement, the annular chamber communicates with the filling stage cylinder by way of a pressure line which may be connected to the supply reservoir by a valve operable by the pressure in the booster chamber. This connection is brought about when the pressure in the booster chamber drops below a predetermined value. In this way the filling stage cylinder will be actuated by the pressure medium displaced out of the annular chamber only when the required boosting pressure is available in the booster chamber. If these is a failure of the boosting pressure due to some trouble and if the braking pressure required for the brake application exclusively has to be generated by the foot pressure acting on the brake pedal, the filling stage cylinder will remain unoperated and the pressure medium displaced out of the annular chamber will flow to the supply reservoir, unpressurized. Thus, the filling stage cylinder will effective only in case of a brake application assisted by the boosted pressure, while in the event of failure of the boosting pressure the braking pressure will be generated by means of the master brake cylinder alone while accepting a larger pedal travel. This known braking pressure generator is very expensive and intricate. It further is disadvantageous in that the actuation of the filling stage cylinder depends on the operation of a valve.
A braking pressure generator is known (German patent application No. P 34 28 136.3 published Feb. 13, 1986 and corresponding to U.S. application Ser. No. 758,306 filed July 24, 1985) which comprises a power booster and master brake cylinder where, on the side facing the master brake cylinder, the booster piston has a smaller diameter portion confining a secondary cylinder chamber designed as annular chamber and reduced in size by the movement of the booster piston upon the application of the brake. The secondary cylinder chamber is connected to a valve which is controllable by the auxiliary pressure of the hydraulic power booster and which in case of lacking auxiliary pressure serves to connect the secondary cylinder chamber with an unpressurized supply reservoir and which in case of existing auxiliary pressure serves to connect the secondary cylinder chamber with a working chamber of the master cylinder. This known braking pressure generator also is expensive and intricate as a hydraulically actuatable valve with a complicated structure in required for the connection of the secondary cylinder chamber. Further, it has proved to be a disadvantage that, when connecting the secondary cylinder chamber with the working chamber of the master brake cylinder, a pressure surge will ensue which will be felt at the brake pedal and which will ensure when different pressures are prevailing in the working chamber and in the secondary cylinder chamber at the moment of connection.
It is thus an object of this invention to provide a braking pressure generator of the type referred to above which is particularly simple and reliable and which avoids pressure surges which might be felt at the brake pedal.